Demodulating apparatus

ABSTRACT

Demodulating apparatus for indicating the difference in depth of modulating of a carrier oscillation by two modulating signals of different frequencies f1 and f2 comprising an aerial with a detector which provides an input to two switching modulators. A first variable-frequency oscillator providing a first reference signal of substantially the same frequency and phase as the signal of frequency f1 is connected to an input of one of the modulators. A second variable-frequency oscillator providing a second reference signal of substantially the same frequency and phase as the signal of frequency f2 is connected to an input of the other modulators. The outputs of the modulators are connected through low-pass filters to a differential amplifier with an associated meter for indicating the difference in the magnitudes of the d.c. components of the outputs of the two modulators.

iJite States aten ansiield et ai.

[451 Sept. 12, 1972 DEMODULATING APPARATUS 3,384,824 5/1968 Kennier..32S/49 [72] Inventors: Michael Patrick Mansfield, 3,171,127 2/1965Asterakl et a]. ..325/307 t t H Pran fi fiis i gg fi g gneu PrimaryExaminer-Robert L. Griffin Assistant Examiner-Barry Leibowitz AsslgneelCm? Limited, Harlow, Attorney-Dike, Bronstein, Roberts & Cushman sex,England 22 Filed: April 22, 1970 [57] ABSTRACT Demodulatin a arat s forindie tin the differen e 21 1. 8 PP a g c 1 App No in depth ofmodulating of a carrier oscillation by two modulating signals ofdifferent frequencies f and f [30] Foreign Application Priority Datacomprising an aerial with a detector which provides an April 22 1969Great Britain 20 607/69 input to two switching modulators. A firstvariablefrequency oscillator providing a first reference signal 52 us.CI. ..325/363 343/108 325/60 substantially the Same freqmcy and Phase as[51] Int Cl 6 1/00 signal of frequency f is connected to an input of one[58] Fiend 60 307 of the modulators. A second variable-frequency oscil-325/329 435 5 3 lator providing a second reference signal of substan- 5tially the same frequency and phase as the signal of frequency f isconnected to an input of the other modulators. The outputs of themodulators are con- [56] References Cited nected through low-passfilters to a differential ampli- UNITED STATES PATENTS fier with anassociated meter for indicating the difference in the ma nitudes of thedc com onents of 3,332,081 7/1967 Lunn et al. ..343/l08 the outputs ofthe a modulatom p 2,924,706 2/ 1960 Sassler ..325/329 3,482,247 12/1969Thompson ..343/ 108 6 Claims, 2 Drawing Figures .c-24 n MULT/Pl/Efl I27212? L //v PHASE VAR/48H fRfQY PHASE OSCILLATOR SH/FTER ,5 'INGit/ADRATURE' ,7 18 of 75C 70R DIFFERENCE Ii MULT/PL/fR I I flit {2 ,5AWL/R 1 IN QUADRATURE 26 v 23 i ig fig PHASE SHIFTER 19 20 p //v PHASELOW-PASS MULT/PL/ER 114 FM TER 25 DEMODULATING APPARATUS The presentinvention relates to a demodulating apparatus for indicating thedifference in depths of modulation of two components of a compositesignal. The apparatus can be fitted to aircraft for use with anInstrument Landing System (I.L.S.)

In the known I.L.S. two carriers of the same frequency'are modulatedwith tones of different frequencies. The carriers are transmitted innarrow beams at small angles to the runway and to opposite sides of theprescribed path of approach for an aircraft. The difference in thedepths of modulation of the carriers with the two tones relative to thecombined carrier amplitude is proportional to the angular distance fromthe correct course at a given distance from the airfield. There is nodifference in the depth of modulation on the correct line of approach.

The present invention provides an apparatus for indicating thedifference in depths of modulation of a carrier oscillation by twomodulating signals of different frequencies f and f including an inputwhich is connected to two multipliers, means for providing to one of themultipliers a first reference signal of substantially the same frequencyand phase as the signal of frequency 1",, means for, providing to theother multiplier a second reference signal of substantially the samefrequency and phase as the signal f the first and second referencesignals being of the same constant amplitude, and means for providing anindication of the difference in the magnitudes of the d.c. components ofv the outputs of the two multipliers.

In using such apparatus in an aircraft when landing using the I.L.S. thepilot steers his aircraft in such a way to tend to maintain .theindication from the apparatus at zero.

An embodiment of the invention will now be described by way of examplewith reference to the accompanying drawing of which the FIG. 1 is ablock diagram of the embodiment and FIG. 2 is a block diagram of part ofthe embodiment of FIG. 1 in more detail.

An aerial 11 is connected to a detector 12 which provides a common inputto two multipliers 13 and 14.

A reference signal for a first 13 of the multipliers is provided by avariable-frequency oscillator 15 which is connected to one input of themultiplier 13 through a phase shifter 16. The phase shifter 16 is alsoconnected to one input of a first subsidiary multiplier 17. The otherinput of the multiplier 17 is provided by the detector l2 and the outputis connected to the variablefrequency oscillator 15 through a low passfilter 18.

Similarly a reference signal for the second multiplier 14 is provided bya variable-frequency oscillator 19 which is connected to one input ofthe multiplier 14 through a phase-shifter 20. The phase-shifter 20 isalso connected to one input of a second subsidiary multiplier 21. Theother input of the multiplier 21 is provided by the detector 12 and theoutput is connected to the variable-frequency oscillator 19 through alow-pass filter 22.

The outputs of the multipliers l3 and 14 are fed to two inputsrespectively of a difference amplifier 23 through low-pass filters 24and 25. The output of the amplifier 23 is connected to a meter 26.

A signal falling on the aerial 11 and consisting of two carriers of thesame frequency and phase and modulated with tones of differentfrequencies Hz and Hz for example), is detected by the detector 12 togive a signal A +m Sin (090! +71 Sin((l) 5oT+ 1 where 0 is the phasedifference between the reference signal and the component. The d.c.component of this output is (ABm/2) cos 0.

The low pass filter 24 allows only the d.c. component of the output fromthe multiplier 13 to pass to the difference amplifier. By locking thereference signal substantially in phase with the component of the inputof the same frequency, 6 becomes very small and cos 0 is approximatelyunity. The input to the difference amplifier is then approximatelyABm/Z.

Similarly, in the multiplier 14 the input is multiplied with a referencesignal of the same frequency and phase as the 150 Hz component of theinput, sin t (b). The amplitude of this reference signal, B, is the sameas the amplitude of the reference signal fed to the multiplier 13. Thed.c. component of the output of the multiplier 14 which passes throughthe low-pass filter 25 to the second input of the difference amplifier23 is ABn 2.

The difference amplifier 23 produces an output which is proportional to(m n), the difference in depth of modulation with respect to thecombined carrier amplitude. This output is applied to the meter 26.

The reference signals are produced by the variable frequency oscillators15 and 19. The signals from the oscillators are fed to the multipliers13 and 14 through the phase shifters 16 and 20. There is no phase changein the signals passing to the multipliers l3 and 14 in the phaseshifters, but signals in quadrature with the inputs to the phaseshifters pass from them to the multipliers l7 and 21. The input from thedetector 12 is multiplied by the quadrature signals in the multipliers17 and 21 to give outputs 14+! Sin (090+)! Sin 0150 COS( (09 t and A [l+m sin (n t+n sin (0) 1+ (#18 cos(w t+ d) +8) respectively, where 8 isthe difference in phase between the reference signal and thecorresponding component of the input. The d.c. components of the outputsfrom the modulators 17 and 21 which pass through the low pass filters 18and 22 are proportional to sin 0 and sin 8 respectively. By using highgain in the feedback lowpass networks to the variable frequencyoscillators the difference in the phases 0 and 6 can be made negligiblysmall.

Preferable the oscillators vibrate at a multiple of the frequencyrequired and the phase shifters digitally divide the signals to providethe in-phase and quadrature signals.

For example, in FIG. 2 the variable-frequency oscillator l5 oscillatesat four time the reference frequency. The oscillations are divided bythe phase shifter 16 which is in the form of a two-stage binary counter.Each stage comprises a flip-flop circuit, the input of the second stagebeing fed with one of the outputs of the flip-flop circuit of the firststage. Thus, the input of the second stage is a train of pulses of twicethe reference frequency. The two outputs of the flip-flop circuit of thesecond stage are used to provide the in-phase and quadrature signals.The first output emits a pulse when the count of pulses from the firststage is odd, and the second emits a pulse when the count of pulses iseven. The first output provides the in-phase signals and the secondprovides the quadrature signal.

The multipliers of FIG. 2 comprises shunt-and series connectedfield-effect transistors (F.E.T.) 28 and 29. The shunt-connectedtransistors provide short circuits to earth when they conduct. TheF.E.T.s are driven by respective drivers 30 and 31 which are actuated bythe outputs from the counter. The drivers 30 invert the signals from theoutputs of the counter so that the F.E.T.s 28 operate in antiphase withthe F.E.T.s 29. Thus the multipliers l3 and 17 operate as switchingmodulators. The low-pass filters 24 and 18 allow only the dc. componentsof the outputs of the multipliers to pass. As only components of themultiplying signals from the counters of the same frequency as the tonesmodulating the carrier signal produce d.c. components, only thefundamental sinusoidal components of the square waveform of signals fromthe counters need be considered, and therefore the mathematical analysisgiven above applies.

The output of the low-pass filter 18 is used to control the phase, andhence the frequency of the variablefrequency oscillator 15. The outputof the low-pass filter 24 is fed to the difference amplifier asdescribed above.

In the l.L.S., the aircraft is on course when the depths of modulationof the two tones are equal. When using the apparatus for aircraftlanding, there is no output form he difference amplifier when theaircraft is on course and the meter gives a center reading. On adeviation to one side of the prescribed course a positive signal isproduced by the amplifier 23 an the needle of the meter 26 is deflectedto one side of the center-position. On a deviation to the other side ofthe prescribed course, a negative signal is produced by the amplifier 23and the needle of the meter 26 is deflected to the other side of thecenter-position. The pilot can steer his aircraft along the prescribedcourse by keeping the needle of the meter 26 in the central position.

We claim:

1. An apparatus for indicating the difference in depths of modulation ofan input carrier oscillation signal by two modulating signals ofdifferent frequenciesf' and f comprising:

two multipliers;

said input signal connected to the two multipliers;

means for providing to one of the multipliers a first reference signalof the same frequency and phase as the signal of frequency f,, said onemultiplier prod cing an o utputh vin ad.c. component; means or providingto t e 0 er multiplier a second reference signal of the same frequencyand phase as the signal of frequency f,, the first and second referencesignals being of the same amplitude, said other multiplier producing anoutput having a dc. component;

and means for providing an indication of the difference in themagnitudes of the dc. components of the outputs of the two multipliers.

2. An apparatus as claimed in claim 1 wherein each of said means forproviding a reference signal comprises: a variable frequency oscillatorhaving a frequency-and-phase control and two outputs providing signalsin quadrature with one another; a first of said output being connectedto one of said multipliers; a subsidiary multiplier connected to thesaid input, the secondoutput being connected to the subsidiarymultiplier; and a low-pass filter, an output of said subsidiarymultiplier being connected to the frequency and phase control of theoscillator through the low-pass filter; whereby the output of saidlow-pass filter controls the phase and frequency of said variablefrequency oscillator so as to maintain the output from said oscillatorat the same frequency and phase as the corresponding modulating signalof said input. I

3. An apparatus as claimed in claim 2 wherein the oscillator is arrangedto vibrate at a multiple of the frequency of the reference signal, andin which the means for providing a reference signal further include aphase shifter which digitally divides the oscillation to provide the twooutput signals in quadrature.

4. An apparatus as claimed in claim 1 wherein the input comprises anaerial and a detector.

5. An apparatus as claimed in claim 1 wherein the multipliers areswitching modulators.

6. An apparatus as claimed in claim 1 wherein the means for providing anindication comprise two lowpass filters and a difference amplifier withan associated meter, the two multipliers being connected to thedifference amplifier through the two low-pass filters; whereby said twolow-pass filters derive said do components from the outputs of said twomultipliers and said difference amplifier provides an indication of thedifference between said two d.c. components.

1. An apparatus for indicating the difference in depths of modulation ofan input carrier oscillation signal by two modulating signals ofdifferent frequencies f1 and f2 comprising: two multipliers; said inputsignal connected to the two multipliers; means for providing to one ofthe multipliers a first reference signal of the same frequency and phaseas the signal of frequency f1, said one multiplier producing an outputhaving a d.c. component; means for Providing to the other multiplier asecond reference signal of the same frequency and phase as the signal offrequency f2, the first and second reference signals being of the sameamplitude, said other multiplier producing an output having a d.c.component; and means for providing an indication of the difference inthe magnitudes of the d.c. components of the outputs of the twomultipliers.
 2. An apparatus as claimed in claim 1 wherein each of saidmeans for providing a reference signal comprises: a variable frequencyoscillator having a frequency-and-phase control and two outputsproviding signals in quadrature with one another; a first of said outputbeing connected to one of said multipliers; a subsidiary multiplierconnected to the said input, the second output being connected to thesubsidiary multiplier; and a low-pass filter, an output of saidsubsidiary multiplier being connected to the frequency and phase controlof the oscillator through the low-pass filter; whereby the output ofsaid low-pass filter controls the phase and frequency of said variablefrequency oscillator so as to maintain the output from said oscillatorat the same frequency and phase as the corresponding modulating signalof said input.
 3. An apparatus as claimed in claim 2 wherein theoscillator is arranged to vibrate at a multiple of the frequency of thereference signal, and in which the means for providing a referencesignal further include a phase shifter which digitally divides theoscillation to provide the two output signals in quadrature.
 4. Anapparatus as claimed in claim 1 wherein the input comprises an aerialand a detector.
 5. An apparatus as claimed in claim 1 wherein themultipliers are switching modulators.
 6. An apparatus as claimed inclaim 1 wherein the means for providing an indication comprise twolow-pass filters and a difference amplifier with an associated meter,the two multipliers being connected to the difference amplifier throughthe two low-pass filters; whereby said two low-pass filters derive saidd.c. components from the outputs of said two multipliers and saiddifference amplifier provides an indication of the difference betweensaid two d.c. components.